A pair of RNA-binding proteins controls networks of splicing events contributing to specialization of neural cell types

MAB-5/Hox regulates the Q neuroblast transcriptome, including cwn-1/Wnt, to mediate posterior migration in Caenorhabditis elegans

Neurogenesis involves the precisely coordinated action of genetic programs controlling large-scale neuronal fate specification down to terminal events of neuronal differentiation. The Q neuroblasts in Caenorhabditis elegans, QL on the left and QR on the right, divide, differentiate, and migrate in a similar pattern to produce three neurons each. However, QL on the left migrates posteriorly, and QR on the right migrates anteriorly. The MAB-5/Hox transcription factor is necessary and sufficient for posterior Q lineage migration and is normally expressed only in the QL lineage. To define genes controlled by MAB-5 in the Q cells, fluorescence-activated cell sorting was utilized to isolate populations of Q cells at a time in early L1 larvae when MAB-5 first becomes active. Sorted Q cells from wild-type, mab-5 loss-of-function (lof), and mab-5 gain-of-function (gof) mutants were subject to RNA-seq and differential expression analysis. Genes enriched in Q cells included those involved in cell division, DNA replication, and DNA repair, consist with the neuroblast stem cell identity of the Q cells at this stage. Genes affected by mab-5 included those involved in neurogenesis, neural development, and interaction with the extracellular matrix. cwn-1, which encodes a Wnt signaling molecule, showed a paired response to mab-5 in the Q cells: cwn-1 expression was reduced in mab-5(lof) and increased in mab-5(gof), suggesting that MAB-5 is required for cwn-1 expression in Q cells. MAB-5 is required to prevent anterior migration of the Q lineage while it transcriptionally reprograms the Q lineage for posterior migration. Functional genetic analysis revealed that CWN-1 is required downstream of MAB-5 to inhibit anterior migration of the QL lineage, likely in parallel to EGL-20/Wnt in a noncanonical Wnt pathway. In sum, work here describes a Q cell transcriptome, and a set of genes regulated by MAB-5 in the QL lineage. One of these genes, cwn-1, acts downstream of mab-5 in QL migration, indicating that this gene set includes other genes utilized by MAB-5 to facilitate posterior neuroblast migration.

A pair of RNA binding proteins inhibit ion transporter expression to maintain lifespan

Regulation of lifespan by transcription factors has been well established. More recently, a role for RNA binding proteins (RBPs) in regulating lifespan has also emerged. In both cases, a major challenge is to determine which regulatory targets are functionally responsible for the observed lifespan phenotype. We recently identified a pair of neuronal RBPs, exc-7/ELAVL and mbl-1/Muscleblind, which in Caenorhabditis elegans display synthetic (nonadditive) lifespan defects: single mutants do not affect lifespan, but exc-7; mbl-1 double mutants have strongly reduced lifespan. Such a strong synthetic phenotype represented an opportunity to use transcriptomics to search for potential causative targets that are synthetically regulated. Focus on such genes would allow us to narrow our target search by ignoring the hundreds of genes altered only in single mutants, and provide a shortlist of synthetically regulated candidate targets that might be responsible for the double mutant phenotype. We identified a small handful of genes synthetically dysregulated in double mutants and systematically tested each candidate gene for functional contribution to the exc-7; mbl-1 lifespan phenotype. We identified 1 such gene, the ion transporter nhx-6, which is highly upregulated in double mutants. Overexpression of nhx-6 causes reduced lifespan, and deletion of nhx-6 in an exc-7; mbl-1 background partially restores both lifespan and healthspan. Together, these results reveal that a pair of RBPs mediate lifespan in part by inhibiting expression of an ion transporter, and provide a template for how synthetic phenotypes (including lifespan) can be dissected at the transcriptomic level to reveal potential causative genes.

TDP-1 and FUST-1 co-inhibit exon inclusion and control fertility together with transcriptional regulation

Gene expression is a multistep process and crosstalk among regulatory layers plays an important role in coordinating gene expression. To identify functionally relevant gene expression coordination, we performed a systematic reverse-genetic interaction screen in C. elegans, combining RNA binding protein (RBP) and transcription factor (TF) mutants to generate over 100 RBP;TF double mutants. We identified many unexpected double mutant phenotypes, including two strong genetic interactions between the ALS-related RBPs, fust-1 and tdp-1, and the homeodomain TF ceh-14. Losing any one of these genes alone has no effect on the health of the organism. However, fust-1;ceh-14 and tdp-1;ceh-14 double mutants both exhibit strong temperature-sensitive fertility defects. Both double mutants exhibit defects in gonad morphology, sperm function, and oocyte function. RNA-Seq analysis of double mutants identifies ceh-14 as the main controller of transcript levels, while fust-1 and tdp-1 control splicing through a shared role in exon inhibition. A skipped exon in the polyglutamine-repeat protein pqn-41 is aberrantly included in tdp-1 mutants, and genetically forcing this exon to be skipped in tdp-1;ceh-14 double mutants rescues their fertility. Together our findings identify a novel shared physiological role for fust-1 and tdp-1 in promoting C. elegans fertility and a shared molecular role in exon inhibition.

VISTA: visualizing the spatial transcriptome of the C.elegans nervous system

Summary

Profiling the transcriptomes of single cells without sacrificing spatial information is a major goal of the field of spatial transcriptomics, but current technologies require tradeoffs between single-cell resolution and whole-transcriptome coverage. In one animal species, the nematode worm Caenorhabditis elegans, a comprehensive spatial transcriptome with single-cell resolution is attainable using existing datasets, thanks to the worm's invariant cell lineage and a series of recently generated single cell transcriptomes. Here we present VISTA, which leverages these datasets to provide a visualization of the worm spatial transcriptome, focusing specifically on the nervous system. VISTA allows users to input a query gene and visualize its expression across all neurons in the form of a "spatial heatmap" in which the color of a cell reports the expression level. Underlying gene expression values (in Transcripts Per Million) are displayed when an individual cell is selected. We provide examples of the utility of VISTA for identifying striking new gene expression patterns in specific neurons, and for resolving cellular identities of ambiguous expression patterns generated from in vivo reporter genes. The ability to easily obtain gene-level snapshots of the neuronal spatial transcriptome should facilitate studies on neuron-specific gene expression and regulation and provide a template for the high-resolution spatial transcriptomes the field hopes to obtain for various animal species in the future.

Availability and implementation

VISTA is freely available at the following URL: https://public.tableau.com/app/profile/smu.oit.data.insights/viz/VISTA_16814210566130/VISTA.

A pair of RNA binding proteins inhibit ion transporter expression to maintain lifespan

Regulation of lifespan by transcription factors has been well established. More recently a role for RNA binding proteins (RBPs) in regulating lifespan has also emerged. In both cases, a major challenge is to determine which regulatory targets are functionally responsible for the observed lifespan phenotype. We recently identified a pair of RBPs, exc-7/ELAVL and mbl-1/Muscleblind, which display synthetic (non-additive) lifespan defects: single mutants do not affect lifespan, but exc-7; mbl-1 double mutants have strongly reduced lifespan. Such a strong synthetic phenotype represented an opportunity to use transcriptomics to search for potential causative targets that are synthetically regulated. Focus on such genes would allow us to narrow our target search by ignoring the hundreds of genes altered only in single mutants, and provide a shortlist of synthetically-regulated candidate targets that might be responsible for the double mutant phenotype. We identified a small handful of genes synthetically dysregulated in double mutants and systematically tested each candidate gene for functional contribution to the exc-7; mbl-1 lifespan phenotype. We identified one such gene, the ion transporter nhx-6, which is highly upregulated in double mutants. Overexpression of nhx-6 causes reduced lifespan, and deletion of nhx-6 in an exc-7; mbl-1 background partially restores both lifespan and healthspan. Together, these results reveal that a pair of RBPs mediate lifespan in part by inhibiting expression of an ion transporter, and provide a template for how synthetic phenotypes (including lifespan) can be dissected at the transcriptomic level to reveal potential causative genes.

VISTA: Visualizing the Spatial Transcriptome of the C. elegans Nervous System

Profiling the transcriptomes of single cells without sacrificing spatial information is a major goal of the field of spatial transcriptomics, but current technologies require tradeoffs between single-cell resolution and whole-transcriptome coverage. In one animal species, the nematode worm C. elegans, a comprehensive spatial transcriptome with single-cell resolution is attainable using existing datasets, thanks to the worm's invariant cell lineage and a series of recently-generated single cell transcriptomes. Here we present VISTA, which leverages these datasets to provide a visualization of the worm spatial transcriptome, focusing specifically on the nervous system. VISTA allows users to input a query gene and visualize its expression across all neurons in the form of a "spatial heatmap" in which the color of a cell reports the expression level. Underlying gene expression values (in Transcripts Per Million) are displayed when an individual cell is selected. We provide examples of the utility of VISTA for identifying striking new gene expression patterns in specific neurons, and for resolving cellular identities of ambiguous expression patterns generated from in vivo reporter genes. The ability to easily obtain gene-level snapshots of the neuronal spatial transcriptome should facilitate studies on neuron-specific gene expression and regulation, and provide a template for the high-resolution spatial transcriptomes the field hopes to obtain for various animal species in the future.

Evolution of the Human Brain Can Help Determine Pathophysiology of Neurodevelopmental Disorders

The evolution of humans brought about a co-occurring evolution of the human brain, which is far larger and more complex than that of many other organisms. The brain has evolved characteristically in humans in many respects, including macro-and micro-anatomical changes in the brain structure, changes in gene expression, and cell populations and ratios. These characteristics are essential for the execution of higher functions, such as sociality, language, and cognition, which express humanity, and are thought to have been acquired over evolutionary time. However, with the acquisition of higher functions also comes the risk of the disease in which they fail. This review focuses on human brain evolution and neurodevelopmental disorders (NDDs) and discusses brain development, molecular evolution, and human brain evolution. Discussing the potential for the development and pathophysiology of NDDs acquired by human brain evolution will provide insights into the acquisition and breakdown of higher functions from a new perspective.

Nucleophosmin3 carried by small extracellular vesicles contribute to white adipose tissue browning

Background

Browning of white adipose tissue (WAT) is a particularly appealing target for therapeutics in the treatment of obesity and related metabolic diseases. Although small extracellular vesicles (sEVs) released from adipose tissue (sEVs-AT) have emerged as novel player that regulate systemic metabolism by connecting different organs, the role of specific contents in sEVs-AT played in WAT browning has not been clarified.

Results

We revealed Nucleophosmin3 (NPM3), which was mainly transferred by sEVs derived from brown adipose tissue (sEVs-BAT), was served as a batokine that could induce WAT browning by regulating the stability of PRDM16 mRNA. sEVs-BAT enhanced the expressions of browning related genes in 3T3-L1 preadipocytes and WAT while knocking down of NPM3 in BAT impaired sEVs-BAT mediated WAT browning and weight loss in obesity.

Conclusion

These data provided new insight into the role of NPM3 in regulating the browning of WAT. Our study indicated that a supplement of sEVs-BAT might represent a promising therapeutic strategy to promote thermogenesis and energy expenditure in the future.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01381-1.

Caenorhabditis elegans ETR-1/CELF has broad effects on the muscle cell transcriptome, including genes that regulate translation and neuroblast migration

Migration of neuroblasts and neurons from their birthplace is central to the formation of neural circuits and networks. ETR-1 is the Caenorhabditis elegans homolog of the CELF1 (CUGBP, ELAV-like family 1) RNA-processing factor involved in neuromuscular disorders. etr-1 regulates body wall muscle differentiation. Our previous work showed that etr-1 in muscle has a non-autonomous role in neuronal migration, suggesting that ETR-1 is involved in the production of a signal emanating from body wall muscle that controls neuroblast migration and that interacts with Wnt signaling. etr-1 is extensively alternatively-spliced, and we identified the viable etr-1(lq61) mutant, caused by a stop codon in alternatively-spliced exon 8 and only affecting etr-1 isoforms containing exon 8. We took advantage of viable etr-1(lq61) to identify potential RNA targets of ETR-1 in body wall muscle using a combination of fluorescence activated cell sorting (FACS) of body wall muscles from wild-type and etr-1(lq61) and subsequent RNA-seq. This analysis revealed genes whose splicing and transcript levels were controlled by ETR-1 exon 8 isoforms, and represented a broad spectrum of genes involved in muscle differentiation, myofilament lattice structure, and physiology. Genes with transcripts underrepresented in etr-1(lq61) included those involved in ribosome function and translation, similar to potential CELF1 targets identified in chick cardiomyocytes. This suggests that at least some targets of ETR-1 might be conserved in vertebrates, and that ETR-1 might generally stimulate translation in muscles. As proof-of-principle, a functional analysis of a subset of ETR-1 targets revealed genes involved in AQR and PQR neuronal migration. One such gene, lev-11/tropomyosin, requires ETR-1 for alternative splicing, and another, unc-52/perlecan, requires ETR-1 for the production of long isoforms containing 3' exons. In sum, these studies identified gene targets of ETR-1/CELF1 in muscles, which included genes involved in muscle development and physiology, and genes with novel roles in neuronal migration.

Two-Color Fluorescent Reporters for Analysis of Alternative Splicing

Alternative splicing is a key layer of gene regulation that is frequently modulated in a spatiotemporal manner. As such, it is a major goal to understand the mechanisms controlling alternative splicing in specific cellular contexts. Reporters that recapitulate alternative splicing patterns of endogenous transcripts have served as excellent tools for dissecting regulatory mechanisms of splicing. In this chapter, we describe a two-color fluorescent reporter system that enables the visualization of alternative splicing patterns by microscopy at single-cell resolution in live animals. We present this reporter system in the context of the model nematode C. elegans.

Sensory neuron transcriptomes reveal complex neuron-specific function and regulation of mec-2/Stomatin splicing

The function and identity of a cell is shaped by transcription factors controlling transcriptional networks, and further shaped by RNA binding proteins controlling post-transcriptional networks. To overcome limitations inherent to analysis of sparse single-cell post-transcriptional data, we leverage the invariant Caenorhabditis elegans cell lineage, isolating thousands of identical neuron types from thousands of isogenic individuals. The resulting deep transcriptomes facilitate splicing network analysis due to increased sequencing depth and uniformity. We focus on mechanosensory touch-neuron splicing regulated by MEC-8/RBPMS. We identify a small MEC-8-regulated network, where MEC-8 establishes touch-neuron isoforms differing from default isoforms found in other cells. MEC-8 establishes the canonical long mec-2/Stomatin isoform in touch neurons, but surprisingly the non-canonical short isoform predominates in other neurons, including olfactory neurons, and mec-2 is required for olfaction. Forced endogenous isoform-specific expression reveals that the short isoform functions in olfaction but not mechanosensation. The long isoform is functional in both processes. Remarkably, restoring the long isoform completely rescues mec-8 mutant mechanosensation, indicating a single MEC-8 touch-neuron target is phenotypically relevant. Within the long isoform we identify a cassette exon further diversifying mec-2 into long/extra-long isoforms. Neither is sufficient for mechanosensation. Both are simultaneously required, likely functioning as heteromers to mediate mechanosensation.

An autoregulation loop in fust-1 for circular RNA regulation in Caenorhabditis elegans

Many circular RNAs (circRNAs) are differentially expressed in different tissues or cell types, suggestive of specific factors that regulate their biogenesis. Here, taking advantage of available mutation strains of RNA-binding proteins (RBPs) in Caenorhabditis elegans, I performed a screening of circRNA regulation in 13 conserved RBPs. Among them, loss of FUST-1, the homolog of Fused in Sarcoma (FUS), caused downregulation of multiple circRNAs. By rescue experiments, I confirmed FUST-1 as a circRNA regulator. Through RNA sequencing using circRNA-enriched samples, circRNAs targets regulated by FUST-1 were identified globally, with hundreds of them significantly altered. Furthermore, I showed that FUST-1 regulates circRNA formation with only small to little effect on the cognate linear mRNAs. When recognizing circRNA pre-mRNAs, FUST-1 can affect both exon-skipping and circRNA in the same genes. Moreover, I identified an autoregulation loop in fust-1, where FUST-1, isoform a (FUST-1A) promotes the skipping of exon 5 of its own pre-mRNA, which produces FUST-1, isoform b (FUST-1B) with different N-terminal sequences. FUST-1A is the functional isoform in circRNA regulation. Although FUST-1B has the same functional domains as FUST-1A, it cannot regulate either exon-skipping or circRNA formation. This study provided an in vivo investigation of circRNA regulation, which will be helpful to understand the mechanisms that govern circRNA formation.

A new gene on C. elegans chromosome V

C. elegans was the first animal to have its genome completely sequenced. In the decades since, the genome continues to be actively curated, annotated, and improved. Here we report the discovery of a new gene in a region of the genome that is currently not associated with any annotated gene or feature. We present RNA-seq and RT-PCR evidence that this gene is expressed at detectable levels, and that it is alternatively spliced. The new gene (Y97E10C.2) shares an operon with two upstream genes. We provide RNA-seq and RT-PCR evidence for a missing exon in the upstream gene T05B11.7, as well as an alternatively-spliced exon.

Complexity and graded regulation of neuronal cell-type-specific alternative splicing revealed by single-cell RNA sequencing

The enormous cellular diversity in the mammalian brain, which is highly prototypical and organized in a hierarchical manner, is dictated by cell-type-specific gene-regulatory programs at the molecular level. Although prevalent in the brain, the contribution of alternative splicing (AS) to the molecular diversity across neuronal cell types is just starting to emerge. Here, we systematically investigated AS regulation across over 100 transcriptomically defined neuronal types of the adult mouse cortex using deep single-cell RNA-sequencing data. We found distinct splicing programs between glutamatergic and GABAergic neurons and between subclasses within each neuronal class. These programs consist of overlapping sets of alternative exons showing differential splicing at multiple hierarchical levels. Using an integrative approach, our analysis suggests that RNA-binding proteins (RBPs) Celf1/2, Mbnl2, and Khdrbs3 are preferentially expressed and more active in glutamatergic neurons, while Elavl2 and Qk are preferentially expressed and more active in GABAergic neurons. Importantly, these and additional RBPs also contribute to differential splicing between neuronal subclasses at multiple hierarchical levels, and some RBPs contribute to splicing dynamics that do not conform to the hierarchical structure defined by the transcriptional profiles. Thus, our results suggest graded regulation of AS across neuronal cell types, which may provide a molecular mechanism to specify neuronal identity and function that are orthogonal to established classifications based on transcriptional regulation.

Spliceosomal component PRP-40 is a central regulator of microexon splicing

Microexons (≤27 nt) play critical roles in nervous system development and function but create unique challenges for the splicing machinery. The mechanisms of microexon regulation are therefore of great interest. We performed a genetic screen for alternative splicing regulators in the C. elegans nervous system and identify PRP-40, a core component of the U1 snRNP. RNA-seq reveals that PRP-40 is required for inclusion of alternatively spliced, but not constitutively spliced, exons. PRP-40 is particularly required for inclusion of neuronal microexons, and our data indicate that PRP-40 is a central regulator of microexon splicing. Microexons can be relieved from PRP-40 dependence by artificially increasing exon size or reducing flanking intron size, indicating that PRP-40 is specifically required for microexons surrounded by conventionally sized introns. Knockdown of the orthologous PRPF40A in mouse neuroblastoma cells causes widespread dysregulation of microexons but not conventionally sized exons. PRP-40 regulation of neuronal microexons is therefore a widely conserved phenomenon.

Molecular topography of an entire nervous system

We have produced gene expression profiles of all 302 neurons of the C. elegans nervous system that match the single-cell resolution of its anatomy and wiring diagram. Our results suggest that individual neuron classes can be solely identified by combinatorial expression of specific gene families. For example, each neuron class expresses distinct codes of ∼23 neuropeptide genes and ∼36 neuropeptide receptors, delineating a complex and expansive "wireless" signaling network. To demonstrate the utility of this comprehensive gene expression catalog, we used computational approaches to (1) identify cis-regulatory elements for neuron-specific gene expression and (2) reveal adhesion proteins with potential roles in process placement and synaptic specificity. Our expression data are available at https://cengen.org and can be interrogated at the web application CengenApp. We expect that this neuron-specific directory of gene expression will spur investigations of underlying mechanisms that define anatomy, connectivity, and function throughout the C. elegans nervous system.

Global regulatory features of alternative splicing across tissues and within the nervous system of C. elegans

Alternative splicing plays a major role in shaping tissue-specific transcriptomes. Among the broad tissue types present in metazoans, the central nervous system contains some of the highest levels of alternative splicing. Although many documented examples of splicing differences between broad tissue types exist, there remains much to be understood about the splicing factors and the cis sequence elements controlling tissue and neuron subtype-specific splicing patterns. By using translating ribosome affinity purification coupled with deep-sequencing (TRAP-seq) in Caenorhabditis elegans, we have obtained high coverage profiles of ribosome-associated mRNA for three broad tissue classes (nervous system, muscle, and intestine) and two neuronal subtypes (dopaminergic and serotonergic neurons). We have identified hundreds of splice junctions that exhibit distinct splicing patterns between tissue types or within the nervous system. Alternative splicing events differentially regulated between tissues are more often frame-preserving, are more highly conserved across Caenorhabditis species, and are enriched in specific cis regulatory motifs, when compared with other types of exons. By using this information, we have identified a likely mechanism of splicing repression by the RNA-binding protein UNC-75/CELF via interactions with cis elements that overlap a 5′ splice site. Alternatively spliced exons also overlap more frequently with intrinsically disordered peptide regions than constitutive exons. Moreover, regulated exons are often shorter than constitutive exons but are flanked by longer intron sequences. Among these tissue-regulated exons are several highly conserved microexons <27 nt in length. Collectively, our results indicate a rich layer of tissue-specific gene regulation at the level of alternative splicing in C. elegans that parallels the evolutionary forces and constraints observed across metazoa.

RNA-binding proteins balance brain function in health and disease

Posttranscriptional gene expression including splicing, RNA transport, translation, and RNA decay provides an important regulatory layer in many if not all molecular pathways. Research in the last decades has positioned RNA-binding proteins (RBPs) right in the center of posttranscriptional gene regulation. Here, we propose interdependent networks of RBPs to regulate complex pathways within the central nervous system (CNS). These are involved in multiple aspects of neuronal development and functioning, including higher cognition. Therefore, it is not sufficient to unravel the individual contribution of a single RBP and its consequences but rather to study and understand the tight interplay between different RBPs. In this review, we summarize recent findings in the field of RBP biology and discuss the complex interplay between different RBPs. Second, we emphasize the underlying dynamics within an RBP network and how this might regulate key processes such as neurogenesis, synaptic transmission, and synaptic plasticity. Importantly, we envision that dysfunction of specific RBPs could lead to perturbation within the RBP network. This would have direct and indirect (compensatory) effects in mRNA binding and translational control leading to global changes in cellular expression programs in general and in synaptic plasticity in particular. Therefore, we focus on RBP dysfunction and how this might cause neuropsychiatric and neurodegenerative disorders. Based on recent findings, we propose that alterations in the entire regulatory RBP network might account for phenotypic dysfunctions observed in complex diseases including neurodegeneration, epilepsy, and autism spectrum disorders.

In vivo Reporters for Visualizing Alternative Splicing of Hormonal Genes

Rapid progress in plant molecular biology in recent years has uncovered the main players in hormonal pathways and characterized transcriptomic networks associated with hormonal response. However, the role of RNA processing, in particular alternative splicing (AS), remains largely unexplored. Here, using example genes involved in cytokinin signaling, brassinosteroid synthesis and auxin transport, we present a set of reporters devised to visualize their AS events in vivo. These reporters show a differential tissue-specific expression of certain transcripts and reveal that expression of some of the them can be changed by the application of the exogenous hormone. Finally, based on the characterized AS event of the PIN7 auxin efflux carrier, we designed a system that allows a rapid genetic screening for the factors upstream of this AS event. Our innovative toolset can be therefore highly useful for exploring novel regulatory nodes of hormonal pathways and potentially helpful for plant researchers focusing on developmental aspects of AS.

mRNA Editing, Processing and Quality Control in Caenorhabditis elegans

While DNA serves as the blueprint of life, the distinct functions of each cell are determined by the dynamic expression of genes from the static genome. The amount and specific sequences of RNAs expressed in a given cell involves a number of regulated processes including RNA synthesis (transcription), processing, splicing, modification, polyadenylation, stability, translation, and degradation. As errors during mRNA production can create gene products that are deleterious to the organism, quality control mechanisms exist to survey and remove errors in mRNA expression and processing. Here, we will provide an overview of mRNA processing and quality control mechanisms that occur in Caenorhabditis elegans, with a focus on those that occur on protein-coding genes after transcription initiation. In addition, we will describe the genetic and technical approaches that have allowed studies in C. elegans to reveal important mechanistic insight into these processes.

Alternative splicing programming of axon formation

Alternative pre-mRNA splicing generates multiple mRNA isoforms of different structures and functions from a single gene. While the prevalence of alternative splicing control is widely recognized, and the underlying regulatory mechanisms have long been studied, the physiological relevance and biological necessity for alternative splicing are only slowly being revealed. Significant inroads have been made in the brain, where alternative splicing regulation is particularly pervasive and conserved. Various aspects of brain development and function (from neurogenesis, neuronal migration, synaptogenesis, to the homeostasis of neuronal activity) involve alternative splicing regulation. Recent studies have begun to interrogate the possible role of alternative splicing in axon formation, a neuron-exclusive morphological and functional characteristic. We discuss how alternative splicing plays an instructive role in each step of axon formation. Converging genetic, molecular, and cellular evidence from studies of multiple alternative splicing regulators in different systems shows that a biological process as complicated and unique as axon formation requires highly coordinated and specific alternative splicing events. This article is categorized under: RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Development.

Splicing in a single neuron is coordinately controlled by RNA binding proteins and transcription factors

Single-cell transcriptomes are established by transcription factors (TFs), which determine a cell's gene-expression complement. Post-transcriptional regulation of single-cell transcriptomes, and the RNA binding proteins (RBPs) responsible, are more technically challenging to determine, and combinatorial TF-RBP coordination of single-cell transcriptomes remains unexplored. We used fluorescent reporters to visualize alternative splicing in single Caenorhabditis elegans neurons, identifying complex splicing patterns in the neuronal kinase sad-1. Most neurons express both isoforms, but the ALM mechanosensory neuron expresses only the exon-included isoform, while its developmental sister cell the BDU neuron expresses only the exon-skipped isoform. A cascade of three cell-specific TFs and two RBPs are combinatorially required for sad-1 exon inclusion. Mechanistically, TFs combinatorially ensure expression of RBPs, which interact with sad-1 pre-mRNA. Thus a combinatorial TF-RBP code controls single-neuron sad-1 splicing. Additionally, we find ‘phenotypic convergence,’ previously observed for TFs, also applies to RBPs: different RBP combinations generate similar splicing outcomes in different neurons.

All the cells in the human nervous system contain the same genetic information, and yet there are many kinds of neurons, each with different features and roles in the body. Proteins known as transcription factors help to establish this diversity by switching on different genes in different types of cells.

A mechanism known as RNA splicing, which is regulated by RNA binding proteins, can also provide another layer of regulation. When a gene is switched on, a faithful copy of its sequence is produced in the form of an RNA molecule, which will then be ‘read’ to create a protein. However, the RNA molecules may first be processed to create templates that can differ between cell types: this means that a single gene can code for slightly different proteins, some of them specific to a given cell type. Yet, very little is known about how RNA splicing can generate more diversity in the nervous system.

To investigate, Thompson et al. developed a fluorescent reporter system that helped them track how the RNA of a gene called sad-1 is spliced in individual neurons of the worm Caenorhabditis elegans. This showed that sad-1 was turned on in all neurons, but the particular spliced versions varied widely between different types of nerve cells.

Additional experiments combined old school and cutting-edge genetics technics such as CRISPR/Cas9 to identify the proteins that control the splicing of sad-1 in different kinds of neurons. Despite not directly participating in RNA splicing, a number of transcription factors were shown to be involved. These molecular switches were turning on genes that code for RNA binding proteins differently between types of neurons, which in turn led sad-1 to be spliced according to neuron-specific patterns.

The findings by Thompson et al. could provide some insight into how mammals can establish many types of neurons; however, a technical hurdle stands in the way of this line of research, as it is still difficult to detect splicing in single neurons in these species.

Deterministic splicing of Dscam2 is regulated by Muscleblind

Alternative splicing increases the proteome diversity crucial for establishing the complex circuitry between trillions of neurons. To provide individual cells with different repertoires of protein isoforms, however, this process must be regulated. Previously, we found that the mutually exclusive alternative splicing of Drosophila Dscam2 produces two isoforms (A and B) with unique binding properties. This splicing event is cell type specific, and the transmembrane proteins that it generates are crucial for the development of axons, dendrites, and synapses. Here, we show that Muscleblind (Mbl) controls Dscam2 alternative splicing. Mbl represses isoform A and promotes the selection of isoform B. Mbl mutants exhibit phenotypes also observed in flies engineered to express a single Dscam2 isoform. Consistent with this, mbl expression is cell type specific and correlates with the splicing of isoform B. Our study demonstrates how the regulated expression of a splicing factor is sufficient to provide neurons with unique protein isoforms crucial for development.

Rbfox1 Mediates Cell-type-Specific Splicing in Cortical Interneurons

Cortical interneurons display a remarkable diversity in their morphology, physiological properties, and connectivity. Elucidating the molecular determinants underlying this heterogeneity is essential for understanding interneuron development and function. We discovered that alternative splicing differentially regulates the integration of somatostatin- and parvalbumin-expressing interneurons into nascent cortical circuits through the cell-type-specific tailoring of mRNAs. Specifically, we identified a role for the activity-dependent splicing regulator Rbfox1 in the development of cortical interneuron-subtype-specific efferent connectivity. Our work demonstrates that Rbfox1 mediates largely non-overlapping alternative splicing programs within two distinct but related classes of interneurons.

Regulation of Neuronal Differentiation, Function, and Plasticity by Alternative Splicing

Posttranscriptional mechanisms provide powerful means to expand the coding power of genomes. In nervous systems, alternative splicing has emerged as a fundamental mechanism not only for the diversification of protein isoforms but also for the spatiotemporal control of transcripts. Thus, alternative splicing programs play instructive roles in the development of neuronal cell type-specific properties, neuronal growth, self-recognition, synapse specification, and neuronal network function. Here we discuss the most recent genome-wide efforts on mapping RNA codes and RNA-binding proteins for neuronal alternative splicing regulation. We illustrate how alternative splicing shapes key steps of neuronal development, neuronal maturation, and synaptic properties. Finally, we highlight efforts to dissect the spatiotemporal dynamics of alternative splicing and their potential contribution to neuronal plasticity and the mature nervous system.

How alternative splicing affects membrane-trafficking dynamics

The cell biology field has outstanding working knowledge of the fundamentals of membrane-trafficking pathways, which are of critical importance in health and disease. Current challenges include understanding how trafficking pathways are fine-tuned for specialized tissue functions in vivo and during development. In parallel, the ENCODE project and numerous genetic studies have revealed that alternative splicing regulates gene expression in tissues and throughout development at a post-transcriptional level. This Review summarizes recent discoveries demonstrating that alternative splicing affects tissue specialization and membrane-trafficking proteins during development, and examines how this regulation is altered in human disease. We first discuss how alternative splicing of clathrin, SNAREs and BAR-domain proteins influences endocytosis, secretion and membrane dynamics, respectively. We then focus on the role of RNA-binding proteins in the regulation of splicing of membrane-trafficking proteins in health and disease. Overall, our aim is to comprehensively summarize how trafficking is molecularly influenced by alternative splicing and identify future directions centered on its physiological relevance.

SUPPA2: fast, accurate, and uncertainty-aware differential splicing analysis across multiple conditions

Despite the many approaches to study differential splicing from RNA-seq, many challenges remain unsolved, including computing capacity and sequencing depth requirements. Here we present SUPPA2, a new method that addresses these challenges, and enables streamlined analysis across multiple conditions taking into account biological variability. Using experimental and simulated data, we show that SUPPA2 achieves higher accuracy compared to other methods, especially at low sequencing depth and short read length. We use SUPPA2 to identify novel Transformer2-regulated exons, novel microexons induced during differentiation of bipolar neurons, and novel intron retention events during erythroblast differentiation.

Alternative Splicing in Neurogenesis and Brain Development

Alternative splicing of precursor mRNA is an important mechanism that increases transcriptomic and proteomic diversity and also post-transcriptionally regulates mRNA levels. Alternative splicing occurs at high frequency in brain tissues and contributes to every step of nervous system development, including cell-fate decisions, neuronal migration, axon guidance, and synaptogenesis. Genetic manipulation and RNA sequencing have provided insights into the molecular mechanisms underlying the effects of alternative splicing in stem cell self-renewal and neuronal fate specification. Timely expression and perhaps post-translational modification of neuron-specific splicing regulators play important roles in neuronal development. Alternative splicing of many key transcription regulators or epigenetic factors reprograms the transcriptome and hence contributes to stem cell fate determination. During neuronal differentiation, alternative splicing also modulates signaling activity, centriolar dynamics, and metabolic pathways. Moreover, alternative splicing impacts cortical lamination and neuronal development and function. In this review, we focus on recent progress toward understanding the contributions of alternative splicing to neurogenesis and brain development, which has shed light on how splicing defects may cause brain disorders and diseases.

Synthetic Genetic Interaction (CRISPR-SGI) Profiling in Caenorhabditis elegans

Genetic interaction screens are a powerful methodology to establish novel roles for genes and elucidate functional connections between genes. Such studies have been performed to great effect in single-cell organisms such as yeast and E. coli (Schuldiner et al., 2005; Butland et al., 2008; Costanzo et al., 2010), but similar large-scale interaction studies using targeted reverse-genetic deletions in multi-cellular organisms have not been feasible. We developed a CRISPR/Cas9-based method for deleting genes in C. elegans and replacing them with a heterologous fluorescent reporter (Norris et al., 2015). Recently we took advantage of that system to perform a large-scale, reverse genetic screen using null alleles in animals for the first time, focusing on RNA binding protein genes (Norris et al., 2017). This type of approach should be similarly applicable to many other gene classes in C. elegans. Here we detail the protocols involved in generating a library of double mutants and performing medium-throughput competitive fitness assays to test for genetic interactions resulting in fitness changes.

Regulated Alternative Splicing of Drosophila Dscam2 Is Necessary for Attaining the Appropriate Number of Photoreceptor Synapses

How the brain makes trillions of synaptic connections using a genome of only 20,000 genes is a major question in modern neuroscience. Alternative splicing is one mechanism that can increase the number of proteins produced by each gene, but its role in regulating synapse formation is poorly understood. In Drosophila, photoreceptors form a synapse with multiple postsynaptic elements including lamina neurons L1 and L2. L1 and L2 express distinct isoforms of the homophilic repulsive protein Dscam2, and since these isoforms cannot bind to each other, cell-specific expression has been proposed to be necessary for preventing repulsive interactions that could disrupt the synapse. Here, we show that the number of synapses are reduced in flies that express only one isoform, and L1 and L2 dendritic morphology is perturbed. We propose that these defects result from inappropriate interactions between L1 and L2 dendrites. We conclude that regulated Dscam2 alternative splicing is necessary for the proper assembly of photoreceptor synapses.

Alternative splicing and the evolution of phenotypic novelty

Alternative splicing, a mechanism of post-transcriptional RNA processing whereby a single gene can encode multiple distinct transcripts, has been proposed to underlie morphological innovations in multicellular organisms. Genes with developmental functions are enriched for alternative splicing events, suggestive of a contribution of alternative splicing to developmental programmes. The role of alternative splicing as a source of transcript diversification has previously been compared to that of gene duplication, with the relationship between the two extensively explored. Alternative splicing is reduced following gene duplication with the retention of duplicate copies higher for genes which were alternatively spliced prior to duplication. Furthermore, and unlike the case for overall gene number, the proportion of alternatively spliced genes has also increased in line with the evolutionary diversification of cell types, suggesting alternative splicing may contribute to the complexity of developmental programmes. Together these observations suggest a prominent role for alternative splicing as a source of functional innovation. However, it is unknown whether the proliferation of alternative splicing events indeed reflects a functional expansion of the transcriptome or instead results from weaker selection acting on larger species, which tend to have a higher number of cell types and lower population sizes.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

The combinatorial control of alternative splicing in C. elegans

Normal development requires the right splice variants to be made in the right tissues at the right time. The core splicing machinery is engaged in all splicing events, but which precise splice variant is made requires the choice between alternative splice sites—for this to occur, a set of splicing factors (SFs) must recognize and bind to short RNA motifs in the pre-mRNA. In C. elegans, there is known to be extensive variation in splicing patterns across development, but little is known about the targets of each SF or how multiple SFs combine to regulate splicing. Here we combine RNA-seq with in vitro binding assays to study how 4 different C. elegans SFs, ASD-1, FOX-1, MEC-8, and EXC-7, regulate splicing. The 4 SFs chosen all have well-characterised biology and well-studied loss-of-function genetic alleles, and all contain RRM domains. Intriguingly, while the SFs we examined have varied roles in C. elegans development, they show an unexpectedly high overlap in their targets. We also find that binding sites for these SFs occur on the same pre-mRNAs more frequently than expected suggesting extensive combinatorial control of splicing. We confirm that regulation of splicing by multiple SFs is often combinatorial and show that this is functionally significant. We also find that SFs appear to combine to affect splicing in two modes—they either bind in close proximity within the same intron or they appear to bind to separate regions of the intron in a conserved order. Finally, we find that the genes whose splicing are regulated by multiple SFs are highly enriched for genes involved in the cytoskeleton and in ion channels that are key for neurotransmission. Together, this shows that specific classes of genes have complex combinatorial regulation of splicing and that this combinatorial regulation is critical for normal development to occur.

Alternative splicing (AS) is a highly regulated process that is crucial for normal development. It requires the core splicing machinery, but the specific choice of splice site during AS is controlled by splicing factors (SFs) such as ELAV or RBFOX proteins that bind to specific sequences in pre-mRNAs to regulate usage of different splice sites. AS varies across the C. elegans life cycle and here we study how diverse SFs combine to regulate AS during C. elegans development. We selected 4 RRM-containing SFs that are all well studied and that have well-characterised loss-of-function genetic alleles. We find that these SFs regulate many of the same targets, and that combinatorial interactions between these SFs affect both individual splicing events and organism-level phenotypes including specific effects on the neuromuscular system. We further show that SFs combine to regulate splicing of an individual pre-mRNA in two distinct modes—either by binding in close proximity or by binding in a defined order on the pre-mRNA. Finally, we find that the genes whose splicing are most likely to be regulated by multiple SFs are genes that are required for the proper function of the neuromuscular system. These genes are also most likely to have changing AS patterns across development, suggesting that their splicing regulation is highly complex and developmentally regulated. Taken together, our data show that the precise splice variant expressed at any point in development is often the outcome of regulation by multiple SFs.

CRISPR-mediated genetic interaction profiling identifies RNA binding proteins controlling metazoan fitness

Genetic interaction screens have aided our understanding of complex genetic traits, diseases, and biological pathways. However, approaches for synthetic genetic analysis with null-alleles in metazoans have not been feasible. Here, we present a CRISPR/Cas9-based Synthetic Genetic Interaction (CRISPR-SGI) approach enabling systematic double-mutant generation. Applying this technique in Caenorhabditis elegans, we comprehensively screened interactions within a set of 14 conserved RNA binding protein genes, generating all possible single and double mutants. Many double mutants displayed fitness defects, revealing synthetic interactions. For one interaction between the MBNL1/2 ortholog mbl-1 and the ELAVL ortholog exc-7, double mutants displayed a severely shortened lifespan. Both genes are required for regulating hundreds of transcripts and isoforms, and both may play a critical role in lifespan extension through insulin signaling. Thus, CRISPR-SGI reveals a rich genetic interaction landscape between RNA binding proteins in maintaining organismal health, and will serve as a paradigm applicable to other biological questions.

An emerging model organism Caenorhabditis elegans for alternative pre-mRNA processing in vivo

A nematode Caenorhabditis elegans is an intron-rich organism and up to 25% of its pre-mRNAs are estimated to be alternatively processed. Its compact genomic organization enables construction of fluorescence splicing reporters with intact genomic sequences and visualization of alternative processing patterns of interest in the transparent living animals with single-cell resolution. Genetic analysis with the reporter worms facilitated identification of trans-acting factors and cis-acting elements, which are highly conserved in mammals. Analysis of unspliced and partially spliced pre-mRNAs in vivo raised models for alternative splicing regulation relying on specific order of intron excision. RNA-seq analysis of splicing factor mutants and CLIP-seq analysis of the factors allow global search for target genes in the whole animal. An mRNA surveillance system is not essential for its viability or fertility, allowing analysis of unproductively spliced noncoding mRNAs. These features offer C. elegans as an ideal model organism for elucidating alternative pre-mRNA processing mechanisms in vivo. Examples of isoform-specific functions of alternatively processed genes are summarized. WIREs RNA 2017, 8:e1428. doi: 10.1002/wrna.1428 For further resources related to this article, please visit the WIREs website.

Transcriptomic profile analysis of mouse neural tube development by RNA-Seq

The neural tube is the primordium of the central nervous system (CNS) in which its development is not entirely clear. Understanding the cellular and molecular basis of neural tube development could, therefore, provide vital clues to the mechanism of neural tube defects (NTDs). Here, we investigated the gene expression profiles of three different time points (embryonic day (E) 8.5, 9.5 and 10.5) of mouse neural tube by using RNA-seq approach. About 391 differentially expressed genes (DEGs) were screened during mouse neural tube development, including 45 DEGs involved in CNS development, among which Bmp2, Ascl1, Olig2, Lhx1, Wnt7b and Eomes might play the important roles. Of 45 DEGs, Foxp2, Eomes, Hoxb3, Gpr56, Hap1, Nkx2-1, Sez6l2, Wnt7b, Tbx20, Nfib, Cntn1 and Dcx had different isoforms, and the opposite expression pattern of different isoforms was observed for Gpr56, Nkx2-1 and Sez6l2. In addition, alternative splicing, such as mutually exclusive exon, retained intron, skipped exon and alternative 3' splice site was identified in 10 neural related differentially splicing genes, including Ngrn, Ddr1, Dctn1, Dnmt3b, Ect2, Map2, Mbnl1, Meis2, Vcan and App. Moreover, seven neural splicing factors, such as Nova1/2, nSR100/Srrm4, Elavl3/4, Celf3 and Rbfox1 were differentially expressed during mouse neural tube development. Interestingly, nine DEGs identified above were dysregulated in retinoic acid-induced NTDs model, indicating the possible important role of these genes in NTDs. Taken together, our study provides more comprehensive information on mouse neural tube development, which might provide new insights on NTDs occurrence. © 2017 IUBMB Life, 69(9):706-719, 2017.

Ancient antagonism between CELF and RBFOX families tunes mRNA splicing outcomes

Over 95% of human multi-exon genes undergo alternative splicing, a process important in normal development and often dysregulated in disease. We sought to analyze the global splicing regulatory network of CELF2 in human T cells, a well-studied splicing regulator critical to T cell development and function. By integrating high-throughput sequencing data for binding and splicing quantification with sequence features and probabilistic splicing code models, we find evidence of splicing antagonism between CELF2 and the RBFOX family of splicing factors. We validate this functional antagonism through knockdown and overexpression experiments in human cells and find CELF2 represses RBFOX2 mRNA and protein levels. Because both families of proteins have been implicated in the development and maintenance of neuronal, muscle, and heart tissues, we analyzed publicly available data in these systems. Our analysis suggests global, antagonistic coregulation of splicing by the CELF and RBFOX proteins in mouse muscle and heart in several physiologically relevant targets, including proteins involved in calcium signaling and members of the MEF2 family of transcription factors. Importantly, a number of these coregulated events are aberrantly spliced in mouse models and human patients with diseases that affect these tissues, including heart failure, diabetes, or myotonic dystrophy. Finally, analysis of exons regulated by ancient CELF family homologs in chicken, Drosophila, and Caenorhabditis elegans suggests this antagonism is conserved throughout evolution.

RNA-binding proteins, neural development and the addictions

Transcriptional and post-transcriptional regulation of gene expression defines the neurobiological mechanisms that bridge genetic and environmental risk factors with neurobehavioral dysfunction underlying the addictions. More than 1000 genes in the eukaryotic genome code for multifunctional RNA-binding proteins (RBPs) that can regulate all levels of RNA biogenesis. More than 50% of these RBPs are expressed in the brain where they regulate alternative splicing, transport, localization, stability and translation of RNAs during development and adulthood. Dysfunction of RBPs can exert global effects on their targetomes that underlie neurodegenerative disorders such as Alzheimer's and Parkinson's diseases as well as neurodevelopmental disorders, including autism and schizophrenia. Here, we consider the evidence that RBPs influence key molecular targets, neurodevelopment, synaptic plasticity and neurobehavioral dysfunction underlying the addictions. Increasingly well-powered genome-wide association studies in humans and mammalian model organisms combined with ever more precise transcriptomic and proteomic approaches will continue to uncover novel and possibly selective roles for RBPs in the addictions. Key challenges include identifying the biological functions of the dynamic RBP targetomes from specific cell types throughout subcellular space (e.g. the nuclear spliceome vs. the synaptic translatome) and time and manipulating RBP programs through post-transcriptional modifications to prevent or reverse aberrant neurodevelopment and plasticity underlying the addictions.

The neurogenetics of alternative splicing

Alternative precursor-mRNA splicing is a key mechanism for regulating gene expression in mammals and is controlled by specialized RNA-binding proteins. The misregulation of splicing is implicated in multiple neurological disorders. We describe recent mouse genetic studies of alternative splicing that reveal its critical role in both neuronal development and the function of mature neurons. We discuss the challenges in understanding the extensive genetic programmes controlled by proteins that regulate splicing, both during development and in the adult brain.

ELAVL2-regulated transcriptional and splicing networks in human neurons link neurodevelopment and autism

The role of post-transcriptional gene regulation in human brain development and neurodevelopmental disorders remains mostly uncharacterized. ELAV-like RNA-binding proteins (RNAbps) are a family of proteins that regulate several aspects of neuronal function including neuronal excitability and synaptic transmission, both critical to the normal function of the brain in cognition and behavior. Here, we identify the downstream neuronal transcriptional and splicing networks of ELAVL2, an RNAbp with previously unknown function in the brain. Expression of ELAVL2 was reduced in human neurons and RNA-sequencing was utilized to identify networks of differentially expressed and alternatively spliced genes resulting from haploinsufficient levels of ELAVL2. These networks contain a number of autism-relevant genes as well as previously identified targets of other important RNAbps implicated in autism spectrum disorder (ASD) including RBFOX1 and FMRP. ELAVL2-regulated co-expression networks are also enriched for neurodevelopmental and synaptic genes, and include genes with human-specific patterns of expression in the frontal pole. Together, these data suggest that ELAVL2 regulation of transcript expression is critical for neuronal function and clinically relevant to ASD.

CELF RNA binding proteins promote axon regeneration in C. elegans and mammals through alternative splicing of Syntaxins

Axon injury triggers dramatic changes in gene expression. While transcriptional regulation of injury-induced gene expression is widely studied, less is known about the roles of RNA binding proteins (RBPs) in post-transcriptional regulation during axon regeneration. In C. elegans the CELF (CUGBP and Etr-3 Like Factor) family RBP UNC-75 is required for axon regeneration. Using crosslinking immunoprecipitation coupled with deep sequencing (CLIP-seq) we identify a set of genes involved in synaptic transmission as mRNA targets of UNC-75. In particular, we show that UNC-75 regulates alternative splicing of two mRNA isoforms of the SNARE Syntaxin/unc-64. In C. elegans mutants lacking unc-75 or its targets, regenerating axons form growth cones, yet are deficient in extension. Extending these findings to mammalian axon regeneration, we show that mouse Celf2 expression is upregulated after peripheral nerve injury and that Celf2 mutant mice are defective in axon regeneration. Further, mRNAs for several Syntaxins show CELF2 dependent regulation. Our data delineate a post-transcriptional regulatory pathway with a conserved role in regenerative axon extension.

DOI:http://dx.doi.org/10.7554/eLife.16072.001

Nerve cells or neurons carry information around the body along projections known as axons. An injury or trauma, such as a stroke, can damage the axons and lead to permanent disability because the damaged axons fail to regenerate over long distances.

Axon damage triggers large changes in the activity of many genes that promote regeneration. When a gene is active, its DNA is copied to make molecules of messenger RNA (mRNA), which are then used as templates to make proteins. Many mRNAs undergo a process called alternative splicing, in which different combinations of mRNA sections may be removed from the final molecule. This enables a single gene to produce more than one type of protein.

Recent studies point to an important role for so-called RNA binding proteins in regulating the alternative splicing process. An RNA binding protein called UNC-75 in a worm known as Caenorhabditis elegans has previously been shown to be involved in axon regeneration, but it was not clear how UNC-75 acts on neurons. Here, Chen et al. combined a technique called CLIP-seq (Cross-linking ImmunoPrecipitation-deep sequencing) with genetic testing to identify the mRNAs that UNC-75 regulates during axon regeneration.

The experiments found a set of C. elegans genes required for information to pass between neurons whose mRNAs are also targeted by UNC-75. Many of these genes are also required for axon regeneration. Chen et al. studied one of the mRNA targets – which encodes a protein called syntaxin – in more detail and found that the syntaxin mRNA is required for regenerating axons over long distances. UNC-75 alternatively splices this mRNA to produce a particular form of syntaxin that is mainly found in neurons. Mutant worms that lack either UNC-75 or syntaxin are unable to properly regenerate axons over long distances.

Further experiments show that a mouse protein known as CELF2 that is equivalent to worm UNC-75 plays a similar role in regenerating axons. Moreover, mouse CELF2 restores the ability of worm neurons that lack UNC-75 to regenerate. Like worm UNC-75, the mouse protein is also involved in alternative splicing of syntaxin. The next step is to examine the other mRNA targets of UNC-75 to find out what role they play in axon regeneration and other processes in neurons.

DOI:http://dx.doi.org/10.7554/eLife.16072.002

Splicing factors control C. elegans behavioural learning in a single neuron by producing DAF-2c receptor

Alternative splicing generates protein diversity essential for neuronal properties. However, the precise mechanisms underlying this process and its relevance to physiological and behavioural functions are poorly understood. To address these issues, we focused on a cassette exon of the Caenorhabditis elegans insulin receptor gene daf-2, whose proper variant expression in the taste receptor neuron ASER is critical for taste-avoidance learning. We show that inclusion of daf-2 exon 11.5 is restricted to specific neuron types, including ASER, and is controlled by a combinatorial action of evolutionarily conserved alternative splicing factors, RBFOX, CELF and PTB families of proteins. Mutations of these factors cause a learning defect, and this defect is relieved by DAF-2c (exon 11.5+) isoform expression only in a single neuron ASER. Our results provide evidence that alternative splicing regulation of a single critical gene in a single critical neuron is essential for learning ability in an organism.

Little is known about the molecular mechanisms regulating neuron-specific alternative splicing. Here, the authors identify a combination of RNA-binding proteins regulating neuron-specific expression of the C. elegans insulin receptor isoform DAF-2c and find disrupting these factors leads to learning deficits.

Control of neuronal synapse specification by a highly dedicated alternative splicing program

Alternative RNA splicing represents a central mechanism for expanding the coding power of genomes. Individual RNA-binding proteins can control alternative splicing choices in hundreds of RNA transcripts, thereby tuning amounts and functions of large numbers of cellular proteins. We found that the RNA-binding protein SLM2 is essential for functional specification of glutamatergic synapses in the mouse hippocampus. Genome-wide mapping revealed a markedly selective SLM2-dependent splicing program primarily consisting of only a few target messenger RNAs that encode synaptic proteins. Genetic correction of a single SLM2-dependent target exon in the synaptic recognition molecule neurexin-1 was sufficient to rescue synaptic plasticity and behavioral defects in Slm2 knockout mice. These findings uncover a highly selective alternative splicing program that specifies synaptic properties in the central nervous system.

Spatio-temporal regulations and functions of neuronal alternative RNA splicing in developing and adult brains

Alternative pre-mRNA splicing is a fundamental mechanism that generates molecular diversity from a single gene. In the central nervous system (CNS), key neural developmental steps are thought to be controlled by alternative splicing decisions, including the molecular diversity underlying synaptic wiring, plasticity, and remodeling. Significant progress has been made in understanding the molecular mechanisms and functions of alternative pre-mRNA splicing in neurons through studies in invertebrate systems; however, recent studies have begun to uncover the potential role of neuronal alternative splicing in the mammalian CNS. This article provides an overview of recent findings regarding the regulation and function of neuronal alternative splicing. In particular, we focus on the spatio-temporal regulation of neurexin, a synaptic adhesion molecule, by neuronal cell type-specific factors and neuronal activity, which are thought to be especially important for characterizing neural development and function within the mammalian CNS. Notably, there is increasing evidence that implicates the dysregulation of neuronal splicing events in several neurological disorders. Therefore, understanding the detailed mechanisms of neuronal alternative splicing in the mammalian CNS may provide plausible treatment strategies for these diseases.

RNA regulation went wrong in neurodevelopmental disorders: The example of Msi/Elavl RNA binding proteins

RNA regulation participates in many aspects of brain development. There is substantial evidence that RNA dysregulation is critical in the pathogenesis of neurodevelopmental disorders, neurological diseases, and cancer. Several gene families encode RNA-binding proteins (RNABPs) that bind directly to RNA and orchestrate the post-transcriptional regulation of gene expression, including pre-mRNA splicing, stability, and poly(A) site usage. Among neural RNABPs, the Elavl and Msi families are the focus of neuronal development research owing to their hierarchical expression pattern: Msi1 is expressed in neural progenitor/stem cells, Elavl2 is expressed in early neuronal progenitors to mature neurons, and Elavl3/4 expression begins slightly later, during cortical neuron development. Traditional biochemical analyses provide mechanistic insight into RNA regulation by these RNABPs, and Drosophila and mouse genetic studies support a relationship between these RNABPs and several neurodevelopmental disorders. In addition, a recent cohort analysis of the human genome shows that genetic mutations and SNPs in these RNABPs are associated with various neurological disorders. Newly emerged technologies assess transcriptome-wide RNA-protein interactions in vivo. These technologies, combined with classical genetics methods, provide new insight into Elavl and Msi, not only with respect to their neurodevelopmental functions, but also their roles in several diseases. We review recent discoveries related to the two RNABP families in brain development and disease.

Alternative splicing of the LIM-homeodomain transcription factor Isl1 in the mouse retina

Islet-1 (Isl1) is a LIM-homeodomain (LIM-HD) transcription factor that functions in a combinatorial manner with other LIM-HD proteins to direct the differentiation of distinct cell types within the central nervous system and many other tissues. A study of pancreatic cell lines showed that Isl1 is alternatively spliced generating a second isoform, Isl1β, which is missing 23 amino acids within the C-terminal region. This study examines the expression of the canonical and alternative Isl1 transcripts across other tissues, in particular, within the retina, where Isl1 is required for the differentiation of multiple neuronal cell types. The alternative splicing of Isl1 is shown to occur in multiple tissues, but the relative abundance of Isl1α and Isl1β expression varies greatly across them. In most tissues, Isl1α is the more abundant transcript, but in others the transcripts are expressed equally, or the alternative splice variant is dominant. Within the retina, differential expression of the two Isl1 transcripts increases as a function of development, with dynamic changes in expression peaking at E16.5 and again at P10. At the cellular level, individual retinal ganglion cells vary in their expression, with a subset of small-to-medium sized cells expressing only the alternative isoform. The functional significance of the difference in protein sequence between the two Isl1 isoforms was also assessed using a luciferase assay, demonstrating that the alternative isoform forms a less effective transcriptional complex for activating gene expression. These results demonstrate the differential presence of the canonical and alternative isoforms of Isl1 amongst retinal ganglion cell classes. As Isl1 participates in the differentiation of multiple cell types within the CNS, the present results support a role for alternative splicing in the establishment of cellular diversity in the developing nervous system.

Regulatory roles of RNA binding proteins in the nervous system of C. elegans

Neurons have evolved to employ many factors involved in the regulation of RNA processing due to their complex cellular compartments. RNA binding proteins (RBPs) are key regulators in transcription, translation, and RNA degradation. Increasing studies have shown that regulatory RNA processing is critical for the establishment, functionality, and maintenance of neural circuits. Recent advances in high-throughput transcriptomics have rapidly expanded our knowledge of the landscape of RNA regulation, but also raised the challenge for mechanistic dissection of the specific roles of RBPs in complex tissues such as the nervous system. The C. elegans genome encodes many RBPs conserved throughout evolution. The rich analytic tools in molecular genetics and simple neural anatomy of C. elegans offer advantages to define functions of genes in vivo at the level of a single cell. Notably, the discovery of microRNAs has had transformative effects to the understanding of neuronal development, circuit plasticity, and neurological diseases. Here we review recent studies unraveling diverse roles of RBPs in the development, function, and plasticity of C. elegans nervous system. We first summarize the general technologies for studying RBPs in C. elegans. We then focus on the roles of several RBPs that control gene- and cell-type specific production of neuronal transcripts.